1 /*
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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23 */
24
25 #ifndef SHARE_OOPS_MARKWORD_HPP
26 #define SHARE_OOPS_MARKWORD_HPP
27
28 #include "metaprogramming/primitiveConversions.hpp"
29 #include "oops/compressedKlass.hpp"
30 #include "oops/oopsHierarchy.hpp"
31 #include "runtime/globals.hpp"
32
33 #include <type_traits>
34
35 // The markWord describes the header of an object.
36 //
37 // Bit-format of an object header (most significant first, big endian layout below):
38 //
39 // 32 bits:
40 // --------
41 // hash:25 ------------>| age:4 self-fwd:1 lock:2 (normal object)
42 //
43 // 64 bits:
44 // --------
45 // unused:22 hash:31 -->| unused_gap:4 age:4 self-fwd:1 lock:2 (normal object)
46 //
47 // 64 bits (with compact headers):
48 // -------------------------------
49 // klass:22 unused_gap:29 hash:2 -->| unused_gap:4 age:4 self-fwd:1 lock:2 (normal object)
50 //
51 // - hash contains the identity hash value: largest value is
52 // 31 bits, see os::random(). Also, 64-bit vm's require
53 // a hash value no bigger than 32 bits because they will not
54 // properly generate a mask larger than that: see library_call.cpp
55 //
56 // - With +UseCompactObjectHeaders:
57 // hashctrl bits indicate if object has been hashed:
58 // 00 - never hashed
59 // 01 - hashed, but not expanded by GC: will recompute hash
60 // 10 - not hashed, but expanded; special state used only by CDS to deal with scratch classes
61 // 11 - hashed and expanded by GC, and hashcode has been installed in hidden field
62 //
63 // When identityHashCode() is called, the transitions work as follows:
64 // 00 - set the hashctrl bits to 01, and compute the identity hash
65 // 01 - recompute idendity hash. When GC encounters 01 when moving an object, it will allocate an extra word, if
66 // necessary, for the object copy, and install 11.
67 // 11 - read hashcode from field
68 //
69 // - the two lock bits are used to describe three states: locked/unlocked and monitor.
70 //
71 // [ptr | 00] locked ptr points to real header on stack (stack-locking in use)
72 // [header | 00] locked locked regular object header (fast-locking in use)
73 // [header | 01] unlocked regular object header
74 // [ptr | 10] monitor inflated lock (header is swapped out, UseObjectMonitorTable == false)
75 // [header | 10] monitor inflated lock (UseObjectMonitorTable == true)
76 // [ptr | 11] marked used to mark an object
77 // [0 ............ 0| 00] inflating inflation in progress (stack-locking in use)
78 //
79 // We assume that stack/thread pointers have the lowest two bits cleared.
80 //
81 // - INFLATING() is a distinguished markword value of all zeros that is
82 // used when inflating an existing stack-lock into an ObjectMonitor.
83 // See below for is_being_inflated() and INFLATING().
84
85 class BasicLock;
86 class ObjectMonitor;
87 class JavaThread;
88 class outputStream;
89
90 class markWord {
91 private:
92 uintptr_t _value;
93
94 public:
95 explicit markWord(uintptr_t value) : _value(value) {}
96
97 markWord() = default; // Doesn't initialize _value.
98
99 // It is critical for performance that this class be trivially
100 // destructable, copyable, and assignable.
101 ~markWord() = default;
102 markWord(const markWord&) = default;
103 markWord& operator=(const markWord&) = default;
104
105 static markWord from_pointer(void* ptr) {
106 return markWord((uintptr_t)ptr);
107 }
108 void* to_pointer() const {
109 return (void*)_value;
110 }
111
112 bool operator==(const markWord& other) const {
113 return _value == other._value;
114 }
115 bool operator!=(const markWord& other) const {
116 return !operator==(other);
117 }
118
119 // Conversion
120 uintptr_t value() const { return _value; }
121 uint32_t value32() const { return (uint32_t)_value; }
122
123 // Constants
124 static const int age_bits = 4;
125 static const int lock_bits = 2;
126 static const int self_fwd_bits = 1;
127 static const int max_hash_bits = BitsPerWord - age_bits - lock_bits - self_fwd_bits;
128 static const int hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits;
129 static const int unused_gap_bits = LP64_ONLY(4) NOT_LP64(0); // Reserved for Valhalla.
130 static const int hashctrl_bits = 2;
131
132 static const int lock_shift = 0;
133 static const int self_fwd_shift = lock_shift + lock_bits;
134 static const int age_shift = self_fwd_shift + self_fwd_bits;
135 static const int hash_shift = age_shift + age_bits + unused_gap_bits;
136 static const int hashctrl_shift = age_shift + age_bits + unused_gap_bits;
137
138 static const uintptr_t lock_mask = right_n_bits(lock_bits);
139 static const uintptr_t lock_mask_in_place = lock_mask << lock_shift;
140 static const uintptr_t self_fwd_mask = right_n_bits(self_fwd_bits);
141 static const uintptr_t self_fwd_mask_in_place = self_fwd_mask << self_fwd_shift;
142 static const uintptr_t age_mask = right_n_bits(age_bits);
143 static const uintptr_t age_mask_in_place = age_mask << age_shift;
144 static const uintptr_t hash_mask = right_n_bits(hash_bits);
145 static const uintptr_t hash_mask_in_place = hash_mask << hash_shift;
146 static const uintptr_t hashctrl_mask = right_n_bits(hashctrl_bits);
147 static const uintptr_t hashctrl_mask_in_place = hashctrl_mask << hashctrl_shift;
148 static const uintptr_t hashctrl_hashed_mask_in_place = ((uintptr_t)1) << hashctrl_shift;
149 static const uintptr_t hashctrl_expanded_mask_in_place = ((uintptr_t)2) << hashctrl_shift;
150
151 #ifdef _LP64
152 // Used only with compact headers:
153 // We store the (narrow) Klass* in the bits 43 to 64.
154
155 // These are for bit-precise extraction of the narrow Klass* from the 64-bit Markword
156 static constexpr int klass_shift = hashctrl_shift + hashctrl_bits;
157 static constexpr int klass_bits = 19;
158 static constexpr uintptr_t klass_mask = right_n_bits(klass_bits);
159 static constexpr uintptr_t klass_mask_in_place = klass_mask << klass_shift;
160 #endif
161
162
163 static const uintptr_t locked_value = 0;
164 static const uintptr_t unlocked_value = 1;
165 static const uintptr_t monitor_value = 2;
166 static const uintptr_t marked_value = 3;
167
168 static const uintptr_t no_hash = 0 ; // no hash value assigned
169 static const uintptr_t no_hash_in_place = (uintptr_t)no_hash << hash_shift;
170 static const uintptr_t no_lock_in_place = unlocked_value;
171
172 static const uint max_age = age_mask;
173
174 // Creates a markWord with all bits set to zero.
175 static markWord zero() { return markWord(uintptr_t(0)); }
176
177 // lock accessors (note that these assume lock_shift == 0)
178 bool is_locked() const {
179 return (mask_bits(value(), lock_mask_in_place) != unlocked_value);
180 }
181 bool is_unlocked() const {
182 return (mask_bits(value(), lock_mask_in_place) == unlocked_value);
183 }
184 bool is_marked() const {
185 return (value() & (self_fwd_mask_in_place | lock_mask_in_place)) > monitor_value;
186 }
187 bool is_forwarded() const {
188 // Returns true for normal forwarded (0b011) and self-forwarded (0b1xx).
189 return mask_bits(value(), lock_mask_in_place | self_fwd_mask_in_place) >= static_cast<intptr_t>(marked_value);
190 }
191 bool is_neutral() const { // Not locked, or marked - a "clean" neutral state
192 return (mask_bits(value(), lock_mask_in_place) == unlocked_value);
193 }
194
195 // Special temporary state of the markWord while being inflated.
196 // Code that looks at mark outside a lock need to take this into account.
197 bool is_being_inflated() const { return (value() == 0); }
198
199 // Distinguished markword value - used when inflating over
200 // an existing stack-lock. 0 indicates the markword is "BUSY".
201 // Lockword mutators that use a LD...CAS idiom should always
202 // check for and avoid overwriting a 0 value installed by some
203 // other thread. (They should spin or block instead. The 0 value
204 // is transient and *should* be short-lived).
205 // Fast-locking does not use INFLATING.
206 static markWord INFLATING() { return zero(); } // inflate-in-progress
207
208 // Should this header be preserved during GC?
209 bool must_be_preserved(const oopDesc* obj) const {
210 return UseCompactObjectHeaders ? !is_unlocked() : (!is_unlocked() || !has_no_hash());
211 }
212
213 // WARNING: The following routines are used EXCLUSIVELY by
214 // synchronization functions. They are not really gc safe.
215 // They must get updated if markWord layout get changed.
216 markWord set_unlocked() const {
217 return markWord(value() | unlocked_value);
218 }
219 bool has_locker() const {
220 assert(LockingMode == LM_LEGACY, "should only be called with legacy stack locking");
221 return (value() & lock_mask_in_place) == locked_value;
222 }
223 BasicLock* locker() const {
224 assert(has_locker(), "check");
225 return (BasicLock*) value();
226 }
227
228 bool is_fast_locked() const {
229 assert(LockingMode == LM_LIGHTWEIGHT, "should only be called with new lightweight locking");
230 return (value() & lock_mask_in_place) == locked_value;
231 }
232 markWord set_fast_locked() const {
233 // Clear the lock_mask_in_place bits to set locked_value:
234 return markWord(value() & ~lock_mask_in_place);
235 }
236
237 bool has_monitor() const {
238 return ((value() & lock_mask_in_place) == monitor_value);
239 }
240 ObjectMonitor* monitor() const {
241 assert(has_monitor(), "check");
242 assert(!UseObjectMonitorTable, "Lightweight locking with OM table does not use markWord for monitors");
243 // Use xor instead of &~ to provide one extra tag-bit check.
244 return (ObjectMonitor*) (value() ^ monitor_value);
245 }
246 bool has_displaced_mark_helper() const {
247 intptr_t lockbits = value() & lock_mask_in_place;
248 if (LockingMode == LM_LIGHTWEIGHT) {
249 return !UseObjectMonitorTable && lockbits == monitor_value;
250 }
251 // monitor (0b10) | stack-locked (0b00)?
252 return (lockbits & unlocked_value) == 0;
253 }
254 markWord displaced_mark_helper() const;
255 void set_displaced_mark_helper(markWord m) const;
256 markWord copy_set_hash(intptr_t hash) const {
257 assert(!UseCompactObjectHeaders, "Do not use with compact i-hash");
258 uintptr_t tmp = value() & (~hash_mask_in_place);
259 tmp |= ((hash & hash_mask) << hash_shift);
260 return markWord(tmp);
261 }
262 // it is only used to be stored into BasicLock as the
263 // indicator that the lock is using heavyweight monitor
264 static markWord unused_mark() {
265 return markWord(marked_value);
266 }
267 // the following two functions create the markWord to be
268 // stored into object header, it encodes monitor info
269 static markWord encode(BasicLock* lock) {
270 return from_pointer(lock);
271 }
272 static markWord encode(ObjectMonitor* monitor) {
273 assert(!UseObjectMonitorTable, "Lightweight locking with OM table does not use markWord for monitors");
274 uintptr_t tmp = (uintptr_t) monitor;
275 return markWord(tmp | monitor_value);
276 }
277
278 markWord set_has_monitor() const {
279 return markWord((value() & ~lock_mask_in_place) | monitor_value);
280 }
281
282 // used to encode pointers during GC
283 markWord clear_lock_bits() const { return markWord(value() & ~lock_mask_in_place); }
284
285 // age operations
286 markWord set_marked() { return markWord((value() & ~lock_mask_in_place) | marked_value); }
287 markWord set_unmarked() { return markWord((value() & ~lock_mask_in_place) | unlocked_value); }
288
289 uint age() const { return (uint) mask_bits(value() >> age_shift, age_mask); }
290 markWord set_age(uint v) const {
291 assert((v & ~age_mask) == 0, "shouldn't overflow age field");
292 return markWord((value() & ~age_mask_in_place) | ((v & age_mask) << age_shift));
293 }
294 markWord incr_age() const { return age() == max_age ? markWord(_value) : set_age(age() + 1); }
295
296 // hash operations
297 intptr_t hash() const {
298 assert(!UseCompactObjectHeaders, "only without compact i-hash");
299 return mask_bits(value() >> hash_shift, hash_mask);
300 }
301
302 bool has_no_hash() const {
303 if (UseCompactObjectHeaders) {
304 return !is_hashed();
305 } else {
306 return hash() == no_hash;
307 }
308 }
309
310 inline bool is_hashed_not_expanded() const {
311 assert(UseCompactObjectHeaders, "only with compact i-hash");
312 return (value() & hashctrl_mask_in_place) == hashctrl_hashed_mask_in_place;
313 }
314 inline markWord set_hashed_not_expanded() const {
315 assert(UseCompactObjectHeaders, "only with compact i-hash");
316 return markWord((value() & ~hashctrl_mask_in_place) | hashctrl_hashed_mask_in_place);
317 }
318
319 inline bool is_hashed_expanded() const {
320 assert(UseCompactObjectHeaders, "only with compact i-hash");
321 return (value() & hashctrl_mask_in_place) == (hashctrl_hashed_mask_in_place | hashctrl_expanded_mask_in_place);
322 }
323 inline markWord set_hashed_expanded() const {
324 assert(UseCompactObjectHeaders, "only with compact i-hash");
325 return markWord((value() & ~hashctrl_mask_in_place) | (hashctrl_hashed_mask_in_place | hashctrl_expanded_mask_in_place));
326 }
327
328 // This is a special hashctrl state (11) that is only used
329 // during CDS archive dumping. There we allocate 'scratch mirrors' for
330 // each real mirror klass. We allocate those scratch mirrors
331 // in a pre-extended form, but without being hashed. When the
332 // real mirror gets hashed, then we turn the scratch mirror into
333 // hashed_moved state, otherwise we leave it in that special state
334 // which indicates that the archived copy will be allocated in the
335 // unhashed form.
336 inline bool is_not_hashed_expanded() const {
337 assert(UseCompactObjectHeaders, "only with compact i-hash");
338 return (value() & hashctrl_mask_in_place) == hashctrl_expanded_mask_in_place;
339 }
340 inline markWord set_not_hashed_expanded() const {
341 assert(UseCompactObjectHeaders, "only with compact i-hash");
342 return markWord((value() & ~hashctrl_mask_in_place) | hashctrl_expanded_mask_in_place);
343 }
344 // Return true when object is either hashed_moved or not_hashed_moved.
345 inline bool is_expanded() const {
346 assert(UseCompactObjectHeaders, "only with compact i-hash");
347 return (value() & hashctrl_expanded_mask_in_place) != 0;
348 }
349 inline bool is_hashed() const {
350 assert(UseCompactObjectHeaders, "only with compact i-hash");
351 return (value() & hashctrl_hashed_mask_in_place) != 0;
352 }
353
354 inline markWord copy_hashctrl_from(markWord m) const {
355 if (UseCompactObjectHeaders) {
356 return markWord((value() & ~hashctrl_mask_in_place) | (m.value() & hashctrl_mask_in_place));
357 } else {
358 return markWord(value());
359 }
360 }
361
362 inline Klass* klass() const;
363 inline Klass* klass_or_null() const;
364 inline Klass* klass_without_asserts() const;
365 inline narrowKlass narrow_klass() const;
366 inline markWord set_narrow_klass(narrowKlass narrow_klass) const;
367
368 #ifdef _LP64
369 inline int array_length() { return checked_cast<int>(value() >> 32); }
370 #endif
371
372 // Prototype mark for initialization
373 static markWord prototype() {
374 if (UseCompactObjectHeaders) {
375 return markWord(no_lock_in_place);
376 } else {
377 return markWord(no_hash_in_place | no_lock_in_place);
378 }
379 }
380
381 // Debugging
382 void print_on(outputStream* st, bool print_monitor_info = true) const;
383
384 // Prepare address of oop for placement into mark
385 inline static markWord encode_pointer_as_mark(void* p) { return from_pointer(p).set_marked(); }
386
387 // Recover address of oop from encoded form used in mark
388 inline void* decode_pointer() const { return (void*)clear_lock_bits().value(); }
389
390 inline bool is_self_forwarded() const {
391 NOT_LP64(assert(LockingMode != LM_LEGACY, "incorrect with LM_LEGACY on 32 bit");)
392 // Match 100, 101, 110 but not 111.
393 return mask_bits(value() + 1, (lock_mask_in_place | self_fwd_mask_in_place)) > 4;
394 }
395
396 inline markWord set_self_forwarded() const {
397 NOT_LP64(assert(LockingMode != LM_LEGACY, "incorrect with LM_LEGACY on 32 bit");)
398 return markWord(value() | self_fwd_mask_in_place);
399 }
400
401 inline markWord unset_self_forwarded() const {
402 NOT_LP64(assert(LockingMode != LM_LEGACY, "incorrect with LM_LEGACY on 32 bit");)
403 return markWord(value() & ~self_fwd_mask_in_place);
404 }
405
406 inline oop forwardee() const {
407 return cast_to_oop(decode_pointer());
408 }
409 };
410
411 // Support atomic operations.
412 template<>
413 struct PrimitiveConversions::Translate<markWord> : public std::true_type {
414 typedef markWord Value;
415 typedef uintptr_t Decayed;
416
417 static Decayed decay(const Value& x) { return x.value(); }
418 static Value recover(Decayed x) { return Value(x); }
419 };
420
421 #endif // SHARE_OOPS_MARKWORD_HPP